usb: dwc2: gadget: Configure the core to enable LPM

Configure core in device mode to support LPM according to
programming guide.
Device will start giving valid responses for LPM tokens.
After this patch device side LPM will start working.

Signed-off-by: Sevak Arakelyan <sevaka@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>